1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concered with a shipping container for transporting nuclear reactor core components, such as a control rod cluster assembly or a burnable absorber rod cluster assembly.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods in a rod cluster control assembly (hereinafter referred to as a control assembly) which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
Also, it is conventional practice to design an excessive amount of neutron flux into the reactor core at start-up so that as the flux is depleted over the life of the core there will still be sufficient reactivity to sustain core operation over a long period of time. In view of this practice, in some reactor applications burnable absorber or poison rods in a burnable absorber cluster assembly (hereinafter referred to as a poison assembly) are inserted within the guide thimbles of some fuel assemblies to assist the control rods in the guide thimbles of other fuel assemblies in maintaining the neutron flux or reactivity of the reactor core relatively constant over its lifetime. The burnable absorber rods, like the control rods, contain neutron absorber material. They differ from the control rods mainly in that they are maintained in stationary positions within the guide thimbles during their period of use in the core.
It is, of course, necessary to transport core components, such as the above-described control assembly and poison assembly, from their location of manufacture to the site of the nuclear reactor. Heretofore, for a number of years, core components have been shipped in wood containers (or boxes). The wood container typically includes an inner and outer box. Recent problems associated with the use of such containers include core components arriving at the site with sawdust and moisture on them. Also, since the rods of the assembly components are supported by plywood spacers, long term storage of components within the containers is not recommended because of potential contamination with halogens and other elements which may leach out of the adhesive in the plywood.
Consequently, a need exists for an alternative approach to construction of shipping containers for transporting reactor core components. A variety of different container constructions appear in the prior art for shipping and storage of radioactive materials. Representative of the prior art containers are those disclosed in U.S. Pat. Nos. to Leebl et al (3,754,141), Boldt (3,828,197), Gablin (3,935,467), Heyer et al (3,971,955), Andersen et al (4,190,160), McMurtry et al (4,218,622) and Botzem et al (4,625,122 and 4,627,956); German patent No. 3,131,126; French patent No. 2,468,979; and Japanese patent No. 239,377. However, none of these prior art constructions appear to provide a suitable alternative approach applicable to shipping or transporting of reactor core components.